England's Luke Donald was just one shot off the clubhouse lead in the delayed second round of the US Open Championship as Merion proved beyond doubt it had stood the test of time.

Donald held a one shot lead over five-time US Open runner-up Phil Mickelson when play was suspended due to darkness yesterday evening following two delays for bad weather totalling more than four hours.

Play resumed at 7:13am this morning and Donald three-putted two of his last five holes to card a two under 68, before returning to Merion's East Course 90 minutes later to shoot a second round 72.

That left him level par and one behind American Billy Horschel, who carded a superb 67 to equal the lowest round of the week so far.

Donald's rollercoaster round featured four birdies and six bogeys, four of them coming in succession from the fourth, and he said: "You're going to make mistakes, I need to try to minimise those mistakes over the next couple of days.

"This is a tough course and it's obviously showing that you don't need a course to be ultra long to make it difficult."

At 6,996 yards, Merion was feared to be too short by modern standards and last staged a US Open in 1981, but Donald added: "I think Merion is holding its own, for sure."

The 35 year old has never recorded a top-ten finish in the US Open and last led a Major at the 2006 US PGA Championship, but always felt the course would suit his game.

"I would love to be a couple of shots better, but certainly I think come the end of round two I'm going to be in a good place," he said.

"I'd take a couple under (as a winning score) right now. I'm excited to be in contention, and have a chance.

"Obviously I haven't played very well in the US Open before, but when I saw this place last week I thought it was a good fit for my game. And obviously it's nice to come here and feel like I'm swinging pretty well and I've got a chance.

"Hopefully I can throw a good one in tomorrow and really be in the mix come Sunday."

Playing partner Lee Westwood had resumed on one under after a double bogey on the 12th yesterday when his third shot to the par four 12th clattered into one of the wicker baskets used instead of a standard flag and rebounded back off the front of the green.

He bogeyed the 17th on his way to an opening 70 and felt he was still in a good position but struggled to a second round of 77 that left him seven over par and facing a long wait to see if that would make the cut.

World Number One Tiger Woods and Rory McIlroy could well be paired together for the third round in succession tomorrow after matching rounds of 73 and 70 left them three over, while the third member of that group - Masters Tournament champion Adam Scott - was alongside Westwood on seven over after rounds of 72 and 75.

Woods had winced in apparent pain from his left arm on several shots out of the rough but refused to go into details about how he had injured his elbow during his victory at the Players Championship last month.

"It is what it is and you move on," he said.

Mickelson began his second round as he began his first, with a bogey, but then carded seven pars in succession, missing from four feet for birdie on the eighth.

At two under par he was one clear of Horschel, who had remarkably hit all 18 greens in regulation during his round of 67.

"It was a great day," said Horschel, whose only bogey came when he three-putted the 115 yard 13th.

"Four birdies at a US Open, I'll take it. I wish I had a couple more though.

"I didn't know I hit every green until I walked off 18. It's a cool thing, but it's not the first time I've hit all 18 greens. I've done it plenty of times in my career. Obviously it's at a US Open, but I think the softness of the greens helped that."

Mickelson's streak of pars ended at ten when he missed from two feet for par on the 12th and also bogeyed the 13th after his approach plugged in a greenside bunker.

That left Horschel, Justin Rose and Steve Stricker in the lead on one under par, Rose and Stricker both two under for their rounds after 11 and 13 holes respectively.